The invention relates in general to a method for preparing pelletized nitrocellulose (PNC), and more particularly to an environmentally friendly method of preparing PNC.
The nitrocellulose (NC) most commonly used by the inventors to prepare PNC (Plastisolor Pelletized Nitrocellulose) is Grade A, Type 1, and has 12.6±0.1% nitrogen content. The NC is prepared from cotton linters and is fibrous in shape. Dry, military grade nitrocellulose is hydroscopic and will absorb between 1 to 3% water when exposed to atmospheres high in relative high humidity. Dry NC is sensitive to impact, friction, spark, and heat. It is much less sensitive when wet with liquids such as alcohol, heptane, petroleum ether, or water.
Water wet NC is classified as a flammable solid by DOT. The transportation of dry NC is not permitted. The preparation of PNC involves changing the fibrous NC into tiny balls (10 to 20 μm) or spheres. Safety is a major concern in the manufacture of PNC. PNC is a mass-detonating explosive when it is dry. This means it can explode instantly when even a small portion is exposed to fire, impact, or electrical spark. Technically it is classed as a “Group 1 Explosive, Class 1, Division 1, DOT Class A”. When PNC is wet with 20% or more water, it is officially classed as a flammable solid, but is very difficult to burn. When PNC is wet with a solvent (for example, alcohol, water or heptane) it is officially classed as a flammable liquid. Solvent wet PNC will burn like the solvent alone.
PNC has been manufactured since the 1950's. Over time a continuous pelletized nitrocellulose (PNC) process was developed and patented. U.S. Pat. Nos. 5,144,020 and 4,590,019 deal with the use of commercial nitrocellulose (NC) and use alcohol-water-organic solvents to produce lacquers. There are three types of commercial NC: RS (11.8 to 12.2% N); SS (spirit-soluble 10.9 to 11.2% N); and AS (11.3 to 11.7% N). RS is soluble in ether/alcohol; SS and AS are alcohol-soluble. The type of NC used in the present invention is military grade (12.2 to 13.8% N). Military grade NC is not soluble in alcohol.
U.S. Pat. Nos. 3,671,515; 3,702,353 and 3,873,517 deal with the preparation of PNC. These patents disclose a process for preparation of a nitromethane/PNC-water emulsion. FIG. 1 is a diagram of a prior art PNC manufacturing process. The prior process includes: 1. Lacquer Preparation; 2. Emulsification; 3. Drowning; 4. Solvent Removal; 5. Product Removal; and 6. Solvent Purification
1. Lacquer Preparation
The lacquer is heated to 140° F. (60° C.). Before opening the lacquer makeup tank, the lacquer tank vent is turned on to draw off the nitromethane fumes. The lacquer is agitated until all the lumps of NC are dissolved, and then diluted to its final concentration. The lacquer is transferred to the lacquer feed tank via the lacquer transfer pump. The lacquer transfer pump should have its discharge valve open at all times to prevent any pressure buildup. Past experience has shown that high nitromethane water content will cause cure, viscosity and particle size problems. This has only been noted when the lacquer has a milky appearance (all water is not dissolved). The nitrocellulose content of the lacquer has been varied in the past and shown to have no impact on cure quality.
2. Emulsification
PNC is formed when the NC/nitromethane lacquer stream is mixed with a water stream and violent agitation in a colloid mill. This forms an emulsion, which is small droplets of the lacquer surrounded by water. This emulsion is next mixed with a large amount of water. This is called “drowning”. The water extracts all the nitromethane solvent from the lacquer droplets leaving the small spherical PNC particles. The lacquer feed rate has been shown to have some effect on the PNC cure quality if it is above a production rate of 40 lbs/hr per emulsification system. The lacquer feed temperature varies between 130° F. and 155° F. (54° C. and 68° C.) due to equipment and control system limitations. The lacquer rate has tended to vary with temperature fluctuations of the lacquer. There is no correlation between lacquer temperature and cure quality. Drown water and emulsification water temperature have been well controlled along with the emulsification water and drown water rates. There is no indication of any correlation between these rates and temperature and cure quality. These rates have been varied extensively without affecting the cure quality.
3. Drowning
The next step is to remove the nitromethane in the drowned emulsion. The drowned emulsion is pumped to the drown column where it is heated from about 120° F. to 150° F. (49° to 66° C.) by the drown column preheater. The drown column must always operate at less than 300 mm Hg pressure. This is to keep the PNC at less than 175° F. (79° C.). If PNC is kept at temperatures above 175° F. (79° C.) it will start to degrade. It will not degrade at rapid rates until it is heated above 205° F. (96° C.). For safety, a limit of 175° F. (79° C.) has been set for normal operations.
4. Solvent Removal
Upon entering the drown column, the nitromethane is boiled away. (This is called “stripping” the feed to the column of nitromethane) The PNC/water flows to the bottom of the column where it is circulated through the drown column reboiler. The reboiler provides the heat for stripping off the remainder of the nitromethane. The nitromethane and water vapor coming off the top of the column are condensed and pumped to a decanter where the water is allowed to separate. The nitromethane is then pumped from the decanter to the nitromethane column. In the nitromethane column, the last of the water is stripped from the nitromethane and pumped back to the decanter, while the purified nitromethane is pumped from the bottom of the column to the nitromethane storage tank.
5. Product Removal
The PNC/water slurry is pumped from the bottom of the drown column to the clarifier to concentrate the slurry. The concentrated slurry is then fed to a centrifuge to separate the water from the PNC. The PNC is discharged from one end and is collected in a conductive plastic bag. The bag is usually changed every half hour, and should never be allowed to collect more than 50 pounds. When starting the clairifier or centrifuge, always wet down first to desensitize any dry PNC that may have accumulated. The PNC/water slurry is fed directly from the clairifier to the vapor body though a spray nozzle. A hot stream of PNC/heptane is recirculated through the vapor body surge tank to boil off the water. A large amount of heptane is also vaporized along with the water. The liquid heptane/PNC overflows from the vapor body to the surge tank where the vapor goes out the top to the heptane recovery system.
6. Solvent Purification
The nitromethane purification process is used to remove water from the wet nitromethane layer. The wet nitromethane normally contains between 2% and 4% water by weight. The dry nitromethane normally contains less than 0.05% water by weight. Since water wet nitrocellulose added in the lacquer preparation area raises the water content of the lacquer to 2.2%, small variations in the nitromethane water content do not have a significant effect on the overall water content of the lacquer. The water content of the nitromethane is not regularly analyzed; however, by monitoring the temperature difference between the upper middle of the column and the column bottom temperature, an indication of the moisture content of the nitromethane can be determined. As long as the bottom temperature is above 145° F. and the temperature difference is less than 25°–30° F., the moisture content is too low to have any impact on the lacquer moisture content. There also may be other impurities in the nitromethane, which may have an impact on the cure. This impurity could be removed by increased boil-up in the column. However, a review of the data does not indicate any correlation between the nitromethane purification operation and the cure of PNC.
7. Tray Drying
PNC is dried by granulating it if necessary and spreading it on trays. Generally all water wet PNC must be granulated prior to spreading. After the PNC is spread, the trays are put into carts and the carts are placed in an oven at 130° F. (54° C.) for up to 9 days. In general, water wet PNC is dried for 7 days until the moisture is less than 0.5% by weight. Heptane wet PNC is dried for 4 days.
Two major disadvantages of the above-described prior process are the use of nitromethane, which is toxic, and the high temperatures used in the drying process. The present invention eliminates the use of nitromethane and the high temperatures used for drying, thereby resulting in a safer, more environmentally friendly process. Further objects, features, and advantages of the invention will become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings.